A method, discussed in DE-OS 26 39 259, employs function planes of the mold which are brought to a desired surface quality mechanically by grinding or polishing. (A function plane is the outer plane of the function layer in contact with the glass.) The function planes must therefore be obtained from function layers which are thick enough for mechanical treatment as well as thick enough to avoid unacceptable deformation during the pressing step.
The manufacture and the removal treatment of such comparatively hard function layers is very expensive. Also, it is difficult to reproducibly form glass articles of high surface quality using this method.
According to the known method, the pressing is isothermal, i.e., the preform and the part of the press interacting with the preform are brought to about the same temperature. In a final state of the pressing, the temperature of the mold and the preform being held under compressive load within the mold is lowered gradually in such a way that the temperatures of the mold and the preform remain approximately equal during the cooling process.
The isothermal pressing is considered to be advantageous because thermal stresses located in the glass-areas lying near the mold which result in micro shifting between the glass surface and the surface of the mold, possibly leading to surface defects of the preform, are avoided. Using similar reasoning, isothermal pressing is also used in other methods, e.g., according to EP 19 342 in the temperature range of the American softening point (10.sup.7.6 dPa.s) or EP 78 658, in which pressing is carried out at comparatively high viscosities.
On the other hand, other methods for the pressing of glass articles for optical purposes are also known, employing conventional pressing of a sufficiently heated preform between function layers of the mold which are at lower temperatures (DE-PS 397 427). Conversely, no definite relation between the temperatures of the preform and the function layers of the mold is discussed. The pressed glass articles manufactured this way, especially lenses, are not suitable for image-forming purposes and are used, e.g., for condensers.
The pressing of glass articles for image-forming purposes is the object of the newer methods and the evolution of the art has shown that in these pressing methods definite temperatures should be preserved. In the previously discussed EP 78 658, a method is described wherein the pressing process takes place at such a high viscosity of the glass, i.e., at such lower temperatures, that the preform can only be deformed minimally during acceptable pressing times. Therefore, the preform has to have a shape corresponding closely to that desired for the final product. This requirement of providing precisely prepared preforms is an additional step in the process. Furthermore, low pressing temperatures require long pressing times and high pressing forces so that overall the method leaves much room for improvement from the standpoint of process economics and product quality assurance.